Spain's Rafa Nadal, and not Roger Federer, is the greatest tennis player of all time, according to eight-time grand slam champion Andre Agassi.

The 32-year-old Federer, with 17 grand slam titles, leads his younger opponent who has 13 majors and is recognised as a complete player on all surfaces.

"I'd put Nadal number one and Federer number two," Agassi told Singapore's Straits Times newspaper, stoking the popular debate.

"Federer separated himself from the field for four years. He separated himself from Andy Roddick and Lleyton Hewitt. Nadal had to deal with Federer, Novak Djokovic, Andy Murray in the golden age of tennis. He has done what he has done and he's not done yet."

Nadal leads Federer 23-10 in all meetings and Agassi said the number of grand slam titles cannot be the sole yardstick to decide the greatest player.

"He has won multiple (majors), every single one (more than once) except the Australian Open - and give him another year on that," Agassi said. "It's just remarkable to me what he has done, and he has done it all during Federer's prime."